Reactor for thermally cracking monofunctional and polyfunctional carbamates

ABSTRACT

A reactor for the thermal cleavage of monofunctional and/or polyfunctional carbamic esters into the corresponding isocyanate and hydroxyl components in the liquid phase having devices for introduction of heat into the reactor, where the devices are heat exchanger plates through which a heat transfer medium flows and have a geometry defined by the ratio of the degassing area to the volume and arrangement of the heating surfaces which makes it possible for the cleavage to be carried out in a two-phase mixture which has a gas content of over 50% by volume.

The invention relates to a reactor for the thermal cleavage ofmonofunctional and/or polyfunctional carbamic esters in the liquid phaseand to a process for the thermal cleavage of carbamic esters in areactor.

The thermal cleavage of carbamic esters to obtain isocyanates has beenknown for a long time. A problem in this process is that the cleavage isreversible, i.e. the isocyanate component and hydroxyl component formedas cleavage products recombine when the hot reaction mixtures arecooled. To prevent the reverse reaction, it is necessary for theisocyanate and hydroxyl components to be separated off from the reactionmixture immediately by means of suitable process engineering measures.

A further problem in the thermal cleavage of carbamic esters is the lossof starting material due to many irreversible decomposition reactions ofthe carbamic esters. For this reason, processes have been developed tosuppress these, in particular by reducing the reaction temperature bythe use of catalysts. Basic catalysts as described, for example, in U.S.Pat. No. 2,692,275 are suitable, but they also catalyze thedecomposition reactions. The aluminum, zinc and tin compounds describedin DE-B 26 35 490 are to be more selective and preferentially acceleratethe cleavage reaction.

A further process improvement which has been proposed, in particular forthe preparation of polyfunctional isocyanates in the case of which thereis an increased risk of polymerization and residue formation, isdilution of the carbamic esters with inert solvents or introduction ofinert solvents into the cleavage reactor, for example as described inEP-A 0 092 738. These measures enable fouling of the surfaces in thereactor to be reduced and by-products can be discharged. Disadvantagesof such an addition of auxiliaries are the resulting increased processengineering outlay and the costs of the auxiliary consumed.

The use of catalytically active solvents, for example dialkylanilines asproposed in U.S. Pat. No. 4,294,774, is also known.

Many proposals for the configuration of the cleavage reactor are known.The cleavage reactors have to take into account the above-describedparticular aspects of the carbamic ester cleavage reaction, inparticular the recombination of the cleavage products, and also suppresssecondary reactors. For this purpose, it is necessary to keep theresidence time of the carbamic esters in the hot zone of the cleavagereactor very short and to remove the cleavage products quickly from thereaction zone. EP-A 0 092 738 describes thin film evaporators andfalling film evaporators as cleavage reactors suitable for this purpose.However, such reactors are expensive for use on a large industrial scaleand the thin film evaporators have the additional disadvantage that theyhave to be provided with a stirrer, i.e. they have moving parts. Thegreatest disadvantage of such reactors is that in the case of thecleavage of polyfunctional carbamic esters they become fouled bypolymerization if they are operated for a period longer than a few days,so that economical operation is not possible.

EP-A 0 524 554 describes the use of simple vessels containing internalsfor introduction of the heat of reaction as cleavage reactors. Thegeometry of the reactor, characterized by the ratio of the degassingarea to the volume and by the arrangement of the heating surfaces, makesit possible for the cleavage to be carried out in a two-phase mixturewhich has a gas content of over 50% by volume. In these reactors, thereaction medium is present in a boiling-like state in which the gasesresulting from the cleavage reaction and starting material vapors ascendthrough the liquid.

Examples which may be mentioned are: Robert evaporators, Herbertevaporators with and without forced circulation, “heating plugvaporizers” having vertical, oblique or horizontal heating plugs,Caddle-type evaporators, tanks with tightly coiled heating coils, andsimilar reactors which have a high heat input into a small volume.

When the boundary conditions described are adhered to, such reactors canbe operated so that polyfunctional carbamic esters can be cleaved toform the corresponding isocyanate and hydroxyl components without thereactors having to be shut down and cleaned during the course ofcontinuous operation for a number of weeks.

The frequently employed variant of a shell-and-tube reactor with theheat transfer medium being passed through the tubes and the reactionmedium being passed through the intermediate space between the tubes ofthe tube bundle suffers from the problem that thermal stresses occur inthe region of the weld for fixing the tubes in tube plates at both endsof the reactor and affect the sealing surfaces, resulting in leaksduring start-up and shutdown and in caked product being formed in thisregion of the reactor.

It is an object of the present invention to provide a reactor which hasdevices for introduction of heat of reaction and displays increasedoperating times of a number of months and can be constructed simply andinexpensively and be extended in a simple manner if required, and doesnot have the disadvantages indicated above.

The achievement of this object starts out from a reactor for the thermalcleavage of monofunctional and/or polyfunctional carbamic esters intothe corresponding isocyanate and hydroxyl components in the liquid phasehaving devices for introduction of heat into the reactor whose geometry,defined by the ratio of the degassing area to the volume of the liquidphase and the arrangement of the heating surfaces, makes it possible forthe cleavage to be carried out in a two-phase mixture having a gascontent of over 50% by volume.

In the reactor of the present invention, the devices are heat exchangerplates through which a heat transfer medium flows.

The reactor of the present invention is operated under processconditions as described in EP-A 0524 554. The reactor is operated insuch a way that the two-phase mixture of boiling liquid and vapor phasecomprising vaporized starting material and the cleavage gases contains aproportion of over 50% by volume of gas.

The volume-based gas content of the two-phase mixture is preferably from50 to 98%, more preferably from 60 to 96% and particularly preferablyfrom 75 to 90%.

To achieve such high gas contents in the two-phase mixture, it is usefulfor the reactors to meet particular geometric requirements.

For example, it is useful for the height of the reaction zone in thereactor to be not less than 0.2 m and not greater than 2 m so as topromote the formation of the gas-rich two-phase mixture. The degassingarea of the reactor, i.e. the free surface of the liquid at which thegas can escape, should be such that the gas velocity at the upper end ofthe reaction zone is not less than 1 m/s and not more than 30 m/s,preferably in the range from 2 m/s to 20 m/s. The heat transfer surfacesshould have dimensions such that the temperature difference betweenheating medium and reaction medium is less than 40° C., but at the sametime the quantity of heat necessary for the strongly endothermicreaction can be introduced into the volume which has been restricted byresidence time requirements.

The reactors are operated fully continuously, both in respect of thefeed and in respect of taking off liquid in order to avoid accumulationof relatively high-boiling by-products. The conversion per path throughthe reactor is in the range from 30 to 95%, preferably from 60 to 90%,conversion of urethane in the reactor feed into isocyanate taken off.

The mean residence times are from 1 to 80 minutes, preferably from 5 to60 minutes, and are defined as the quotient of the liquid contents ofthe reactor under reaction conditions and the reactor contents taken inliquid form from the reactor per minute.

The reaction is carried out without any addition of solvent or diluentand without an inert gas being conveyed through the reactor.

The feed to the reactor is a product mixture which comprises thecarbamic ester and by-products formed in a circulation process in whicha carbamic ester is prepared from an amine with addition of urea and ahydroxyl component and this is then cleaved to form the isocyanate, withpart of the contents of the reactor being taken from the reactor andrecirculated to the first stage of the process. Such by-products can be,inter alia, high molecular weight products, preferably isocyanurate.Surprisingly, it is sufficient for the carbamic ester content of thefeed to the reactor to be from 80 to 90%. A higher purity of thecarbamic ester makes operation of the cleavage reactor easier, sincefouling problems then become less significant. It is also possible touse reactor feeds containing less than 80% of carbamic ester, butfouling problems then increase, particularly when polyfunctionalcarbamic esters are being cleaved. The feed to the cleavage reactor can,if desired, further comprise the catalyst necessary for the cleavage.

An important aspect of the invention is the fact that the carbamic esterdoes not have to be completely vaporized and condensed again to free itof oligomeric by-products before it is fed into the reactor, asdescribed in EP-A 355 433, but instead a certain level of by-products inthe reactor can be tolerated.

The use according to the present invention of heat exchanger plates asdevices for the introduction of heat makes it possible to cleave evencarbamic esters having a relatively high level of impurities and acarbamic ester content of from 70 to 85%.

According to the present invention, heat exchanger plates are used asdevices for introduction of heat in the cleavage reactor.

Heat exchanger plates are devices known in process engineering for heattransfer. They are generally made up of two essentially parallel metalsheets which are joined to one another, in particular welded together,and form an interior space through which a heat transfer medium can bepassed via suitable inlet and outlet lines. To increase the stability,in particular the compressive strength, the plates are frequently weldedtogether at a point and/or along lines in a plurality of places.

Heat exchanger plates are generally used in a stack, i.e. as a pluralityof parallel heat exchanger plates. For use in the reactor of the presentinvention, the heat exchanger plates are preferably made of stainlesssteel, in particular a stainless steel having the material numbers1.45xx, with the two “x”s being able to denote any numbers, preferably astainless steel having one of the material numbers 1.4571, 1.4529,1.4401, 1.4404 or 1.4462. The heat exchanger plates are preferablyprovided with a smooth, in particular polished, for exampleelectropolished, surface.

Due to their geometric shape, the heat exchanger plates force thereaction mixture flowing in the intermediate spaces between the heatexchanger plates to flow in the longitudinal direction. As a result,heat transfer between the reaction mixture and the heat transfer mediumflowing through the heat exchanger plates is poorer than in the case ofa shell-and-tube reactor in which transverse mixing of the reactionmedium is also possible between the tubes through which heat transfermedium flows. For this reason, as a given reaction volume, a cleavagereactor provided with heat exchanger plates requires a larger heattransfer area than does a shell-and-tube cleavage reactor. Thus, forexample, a heat transfer area of 150 m² for a shell-and-tube reactor hasto be increased to 228 m² for a reactor provided with heat exchangerplates at the same reaction volume. However, since the plate reactor issimpler to manufacture, the manufacturing costs are nevertheless only25-30% of the manufacturing costs for the shell-and-tube reactor.

Furthermore, when heat exchanger plates are used, the temperature atwhich the heat transfer medium is introduced and thus the stress on theheating surfaces can be reduced, frequently by up to about 20° C., for agiven reaction volume because of the increased heat transfer area. Thisleads to reduced deposit formation on the surfaces and thus to alengthening of the uninterrupted operating time of the reactor,frequently by about 20%.

The cleavage reactor of the present invention is preferably configuredso that the heat exchanger plates dip partly or completely into theliquid phase and the two-phase reaction mixture.

As heat transfer medium in the heat exchanger plates, preference isgiven to using high-boiling liquids, i.e. liquids having a boiling pointwhich is at least about 40° C. above the cleavage temperature of thecarbamic ester used, in particular a boiling point in the range from 280to 400° C., preferably from 350 to 390° C. Particularly preferred heattransfer media are a Marlotherm® fluid or mixtures of Marlotherm®fluids. Marlotherms® are mixtures of dibenzyltoluenes (Marlotherm®S) ormixtures of isomeric benzyltoluenes (Marlotherm® L).

The heat exchanger plates preferably have dimensions such that the ratioof the total heat transfer area of the heat exchanger plates to thetotal volume of the liquid phase in the reactor is in the range from 10to 320 m²/m³, preferably in the range from 20 to 150 m²/m³, inparticular in the range from 50 to 80 m²/m³.

In a preferred embodiment, the cleavage reactor is a horizontal cylinderin which the heat exchanger plates are configured as segments of acircle which are arranged parallel to one another and perpendicular tothe longitudinal direction of the reactor in the lower half of thereactor.

The segments of a circle are particularly preferably smaller than halfthe cross section of the reactor.

The supports holding the heat exchanger plates are preferably made asvibration-free as possible so as not to interfere with the turbulence inthe reactor. For this purpose, the heat exchanger plates are preferablyattached to inlet and outlet pipes arranged in the longitudinaldirection of the reactor above the heat exchanger plates by means ofcurved pipe sections.

As a result of this particular configuration of the inlet and outlet forthe heat transfer medium, there are no longer any large, heated areas atwhich caking of by-products could take place in the caps at the two endsof the cleavage reactor in the region of the inlet or outlet of the heattransfer medium, as is the case on the heated tube plates ofshell-and-tube reactors.

Furthermore, the cleavage reactor of the present invention which hasheat exchanger plates can readily be adapted to a need for an increasedheat transfer area by installation of additional heating plates andenlargement of the outer wall of the reactor.

The present invention also provides a process for the thermal cleavageof carbamic esters in a cleavage reactor as described above.

The monofunctional or polyfunctional carbamic esters which can be usedare in principle subject to no restrictions.

Likewise, the process is not restricted in respect of the furtherwork-up of the cleavage gases leaving the reactor during the cleavagereaction. These are preferably fractionated by rectification, asdescribed in EP-A 0 524 554.

The process is preferably carried out with no solvent being added to thereaction medium.

The cleavage rector is preferably operated at a pressure of from 2 to200 mbar, particularly preferably at a pressure of from 5 to 100 mbar.

The cleavage reactor is preferably operated so that the velocity of thegases leaving the two-phase reaction medium at the upper end of thereaction zone is from 1 to 30 m/s, preferably from 2 to 20 m/s, when thereaction is carried out at an absolute pressure in the range from 2 mbarand 200 mbar.

The invention is illustrated below with the aid of a drawing and anexample.

FIG. 1 schematically shows a preferred embodiment of a heat exchangerplate in cross section.

FIG. 1 shows a heat exchanger plate 1 having the shape of a circle. Theupper part of the heat exchanger plate 1 is provided with distributionand collection manifolds 2, 3 for the heat transfer medium, which are,by way of example, configured as tubes having a circular cross-section.In the preferred variant shown in FIG. 1, the heat transfer medium isfed in via the outer distribution and collection manifold 2 anddischarged via the inner distribution and collection manifold 3. Thedistribution and collection manifolds 2, 3 are connected to the heatexchanger plate 1 by means of curved pipe sections 4 which ensure thatthe heat exchanger plates 1 are held in place in a virtuallyvibration-free manner. In the cleavage reactor of the present invention,a plurality of heat exchanger plates 1 are arranged one after the otherwith corresponding distribution and collection manifolds 2, 3 and curvedpipe sections 4. The curved pipe sections arranged one after the othercan therefore be referred to as a comb-like support for the heatexchanger plates 1.

A liquid feed stream of 45 kg/h of hexamethylenedi-n-butylurethane wasfed continuously into a horizontal cylindrical reactor having a diameterof 40 cm. The cleavage reaction was carried out at 240° C. in thereaction medium and a pressure of 30 mbar. A liquid stream of 15 kg/hwas taken off continuously from the bottom of the reactor by means of apump. The liquid holdup in the reactor and the pipes was about 10liters. The mean residence time was in the range from 30 to 40 minutes.About 10 kW of thermal power had to be introduced into the reactionmedium to maintain the endothermic reaction and to vaporize the reactionproducts. Marlotherm® S was used as heat transfer medium.

Under the abovementioned conditions, a comparison was made between ashell-and-tube cleavage reactor containing 15 tubes each having adiameter of 1.8 cm and a cleavage reactor according to the presentinvention provided with heat exchanger plates. In the comparativeexperiment using the shell-and-tube reactor, the inlet temperature ofthe Marlotherm® fluid was 260° C. and the heat exchange area was about0.35 m².

In comparison, the reactor according to the present invention wasprovided with 9 heat exchanger plates which were arranged vertically inthe reactor with a spacing between the individual plates of 15 mm. Theinlet temperature of the Marlotherm® fluid was 258° C. and the heatexchange area was about 0.6 m².

The cleavage of the carbamic ester occurred at the same conversion ofabout 95% both in the reactor of the prior art and in the reactoraccording to the present invention. However, chromatographic analysisshowed that the increase in high molecular weight compounds which leadto fouling, in particular allophanates, cyanurates, etc., was about 15%lower when using the cleavage reactor of the present invention providedwith heat exchanger plates. After an operating time of three weeks, nofouling on the heat exchanger surfaces was found in the case of thereactor according to the present invention, in contrast to the reactorof the prior art.

1. A reactor comprising devices for introduction of heat into thereactor, wherein the devices are heat exchanger plates wherein a heattransfer medium flows through said heat exchanger plates, said reactoris capable of carrying out a thermal cleavage of monofunctional and/orpolyfunctional carbamic esters into the corresponding isocyanate andhydroxyl components in a liquid phase without addition of a solvent, andthe reactor is capable of carrying out said cleavage in a two-phasemixture comprising a gas content of over 50% by volume due to thegeometry of said reactor, wherein the geometry is defined by the ratioof the degassing area to the volume of the liquid phase and thearrangement of the heating surfaces, wherein the heat exchanger platesare configured as segments of a circle arranged parallel to one anotherand perpendicular to the longitudinal axis of the reactor in the lowerhalf of the reactor, and wherein the heat exchanger plates are supportedby curved pipe sections in a largely vibration-free manner along feedand distribution pipes arranged in the longitudinal direction of thereactor above the heat exchanger plates.
 2. The reactor as claimed inclaim 1, wherein the heat exchanger plates dip partly or completely intothe liquid phase.
 3. The reactor as claimed in claim 1, wherein the heattransfer medium is a high-boiling liquid.
 4. The reactor as claimed inclaim 3, wherein the high-boiling liquid is a mixture ofdibenzyltoluenes, mixture of isomeric benzyltoluenes, or mixturesthereof.
 5. The reactor as claimed in claim 1, wherein the ratio of thetotal heat transfer area of the heat exchanger plates to the totalvolume of the liquid phase in the reactor is in the range from 10 to 320m²/m³.
 6. The reactor as claimed in claim 5, wherein the ratio of thetotal heat transfer area of the heat exchanger plates to the totalvolume of the liquid phase in the reactor is in the range from 20 to 150m²/m³.
 7. The reactor as claimed in claim 6, wherein the ratio of thetotal heat transfer area of the heat exchanger plates to the totalvolume of the liquid phase in the reactor is in the range from 20 to 80m²/m³.
 8. The reactor as claimed in claim 1, wherein the reactor isconfigured as a horizontal cylinder.
 9. The reactor as claimed in claim8, wherein the segments of a circle are smaller than half the crosssection of the reactor.
 10. The reactor as claimed in claim 9, whereinthe segments of a circle have an area of from 20 to 30% of thecross-sectional area of the reactor.
 11. A process for producingisocyanate and hydroxyl components, the process comprising: thermallycleaving monofunctional and/or polyfunctional carbamic esters into thecorresponding isocyanate and hydroxyl components in the liquid phasewithout addition of a solvent in a two-phase in a reactor wherein saidreactor comprises devices for introduction of heat into the reactor, thedevices are heat exchanger plates wherein a heat transfer medium flowsthrough said heat exchanger plates, and the reactor is capable ofcarrying out said cleaving in a two-phase mixture comprising a gascontent of over 50% by volume due to the geometry of said reactor,wherein the geometry is defined by the ratio of the degassing area tothe volume of the liquid phase and the arrangement of the heatingsurfaces, wherein the heat exchanger plates are configured as segmentsof a circle arranged parallel to one another and perpendicular to thelongitudinal axis of the reactor in the lower half of the reactor, andwherein the heat exchanger plates are supported by curved pipe sectionsin a largely vibration-free manner along feed and distribution pipesarranged in the longitudinal direction of the reactor above the heatexchanger plates.
 12. The process as claimed in claim 11, wherein theprocess is carried out at a pressure of from 2 to 200 mbar in thereactor.
 13. The process as claimed in claim 11, wherein the process iscarried out at a pressure of from 5 to 100 mbar in the reactor.
 14. Theprocess as claimed in claim 11, wherein the velocity of the gasesleaving the two-phase mixture at the upper end of the reaction zone isfrom 1 m/s to 30 m/s.
 15. The process as claimed in claim 14, whereinthe velocity of the gases leaving the two-phase mixture at the upper endof the reaction zone is from 2 m/s to 20 m/s.